Titanium dioxide pigments are nowadays prepared not only by the sulphate process but also by the combustion process in which TiCl.sub.4 and oxygen are directly converted into titanium dioxide pigments by heating to an elevated temperature.
The TiCl.sub.4 required for this purpose is obtained by the chlorination of materials containing titanium, such as ilmenite, leucoxene or rutile, in the presence of carbon. The crude TiCl.sub.4 obtained is contaminated with numerous other chlorides and with chlorine, the main impurities being the chlorides of iron, aluminium and silicon as well as the chlorides and oxychlorides of vanadium.
Removal of these impurities is essential if the TiO.sub.2 pigments are to have a pure white color.
Most of the impurities, such as the chlorides of iron, aluminium and silicon, may be removed by distallation. The distillation of crude TiCl.sub.4 results in the formation of thickened suspensions of the solid metal chlorides and of extremely finely divided residues of crude chlorination products which are difficult to evaporate to dryness, especially if they contain a high proportion of AlCl.sub.3. This distillation therefore entails a high energy consumption and/or losses in TiCl.sub.4 yield.
It would therefore be preferable to separate the solid metal chlorides and the finely divided residues of the solid raw material of the chlorination process by filtration and to dry the filter cake. This method, however, becomes very expensive due to the difficulty of filtering the finely divided solids.
The product obtained after removal of the solid impurities still contains vanadium in the form of VOCl.sub.3 or VCl.sub.4.
The removal of VOCl.sub.3 and VCl.sub.4 from the titanium tetrachloride by distillation is costly due to the similarity of their boiling points.
These compounds are therefore converted into solid, low valency vanadium chlorides by reduction.
A product containing only low vanadium concentrations (so-called pure TiCl.sub.4) may then be obtained by distillation.
Known methods of purifications use, for example, H.sub.2 S (DE-A No. 1 923 479), animal and vegetable oils, fats, waxes, resins and soaps, liquid or gaseous hydrocarbons, oils, fats, alcohols, ketones, organic acids, amines (CH-A No. 365 393, CH-A No. 262 267, DE-C No. 867 544, FR-A No. 1 466 478, FR-A No. 1 460 362), metals and metal salts (BE-A No. 539 078, DE-A No. 1 922 420, DE-B No. 1 271 693, U.S. Pat. Nos. 3,915,364, 2,871,094, 2,753,255, 2,560,424, 2,555,361, 2,530,735 and 2,178,685).
The purification of titanium tetrachloride with special cyclic aliphatic or aromatic compounds (DE-C No. 2 329 045) and with special amines (DE-C No. 2 325 924) is particularly advantageous.
It was an object of the present invention to provide a particularly advantageous process for the purification of TiCl.sub.4 which would not have the disadvantages described above.